1. Field of the Invention
This invention pertains to the use of certain compounds to inhibit or prevent the crosslinking of aqueous xanthan gum in the presence of ferric ion.
2. Description of the Prior Art
Various acids have long been used to increase the permeability of a formation surrounding a wellbore. In these treatments, the acid is normally pumped from the surface through iron pipe under pressure into the formation to dissolve various parts of the formations. Such acid treatments have been particularly effective in calcareous formations and have been effective in increasing the production of oil, gas, brine, or even water wells.
During such acid treatments, the treatment acid dissolves iron scale in pipes and iron-containing minerals in the formation. The dissolved iron normally remains in solution until the acid is spent. Upon spending, ferric hydroxide begins to precipitate and plug the formation. Complete precipitation of ferric hydroxide is reached at a pH of about 3.2. Ferrous hydroxide, being more soluble, does not begin to precipitate until a pH of approximately 7.7 and is not generally a problem.
The deleterious effects of ferric hydroxide in wells was recognized by Grebe in U.S. Pat. No. 2,175,081 as early as 1937. Grebe used a strong acid containing sulphurous acid to counter the precipitation problem.
Numerous other procedures have been proposed for avoiding the ferric hydroxide problem. For example, U.S. Pat. No. 2,175,095 suggests including within the acidizing fluid a material such as lactic acid, ammonium acetate, glycine, glycolic acid, citric acid, or the like, which is capable of preventing the precipitation of iron or aluminum hydroxides at normal precipitation pH values. U.S. Pat. No. 2,335,689 suggests adding an iron sequestering agent, such as a polyhydric phenol within the injected acids. U.S. Pat. No. 3,142,335 suggests the use of a sequestering agent containing a mixture of ingredients that function as a pH buffer, such as citric acid or a citrate salt mixed with acetic or formic acids or their salts. U.S. Pat. No. 3,150,081 suggests using mixtures of hydroxyacetic and citric acids; the mixtures are alleged to be cheaper and more effective than the use of either acid alone.
The most common iron sequestering agents in commercial practice are citric acid, ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), acetic acid, lactic acid, and citric acid/acetic acid mixtures. Data on these sequestering agents is found in the paper by Smith et al., Paper No. SPE 2358, Society of Petroleum Engineers of AIME, (presented Nov. 7-8, 1968).
The problem with most iron sequestering agents is that they are not particularly effective at temperatures beyond about 125.degree.-150.degree. F. Only NTA, citric acid and EDTA have shown any effectiveness at higher temperatures. And of these, EDTA is the only practical sequestering agent because citric acid tends to form insoluble citrates during the course of the well treatment. Such citrates can block the well production almost as effectively as the ferric hydroxide.
The presence of the ferric ions in the treatment acid solution is known to cause other serious problems as well. For example, U.S. Pat. No. 4,096,914 teaches that ferric iron reacts with asphaltenic oil to form insoluble iron-asphaltene compounds. These compounds are insoluble precipitates which likewise plug the formation channels and inhibit production of the desired fluid. The patent teaches that the problem can be solved by incorporating salicylic acid into the treatment acid.
Ferric ion corrosion can also be a problem. Each mole of ferric ion reacts with base metal to form two moles of ferrous ion. Almost any oxidizing source readily converts the ferrous ions to ferric ions, and a vicious circle results.
Additives used to control problems associated with ferric ions in treatment of wells have been called "iron stabilizers" by practitioners in the field.
So the state of the art is such that there is a substantial need to prevent the formation of ferric ions during acid treatment of wells and there is a particular need to prevent the formation and precipitation of ferric compounds as the acid spends. There is also a particular need for aqueous gelled acids which will not crosslink upon spending in the presence of ferric ions. Aqueous gelled acids can be used with a high degree of effectiveness in the acidizing treatment of wells mentioned above.
Various polysaccharides have been used previously as thickeners for aqueous acids, but their performance was less than satisfactory because of the tendency to rapidly degrade with shear and/or high temperatures encountered under conditions of use. Many of these polysaccharides, including xanthan gums, crosslink in the presence of polyvalent metal ions. As an example, aqueous acid gelled with xanthan gum crosslinks readily in the presence of ferric ion at a pH higher than about 1.5.